1. Field of the Invention
The present invention relates to a magnetic recording medium, and more particularly, to a magnetic recording medium having high resistance to oxidation, great durability to repeated still-frame operation, and high storage stability.
2. Description of the Prior Art
Magnetic medium generally consists of a support shaped in a tape or sheet form from poly (ethylene terephthalate) having a magnetic layer applied from a coating paint containing magnetic powder and a binder as two major components. Metallic powder incorporated in the magnetic layer as magnetic powder are superior to iron oxide powder with respect to squareness ratio, saturation magnetization and coercivity. However, metallic powder smaller than 1 .mu.m are not only difficult to disperse but also are easily oxidized and corroded to cause a gradual decrease in the saturation magnetization, storage stability and durability to repeated still-frame operation. Also, metallic powder are highly reactive and are pyrophoric at room temperature. The instability of metallic powder against oxidation is partly due to the inherent properities of the metal, but the principal cause is said to be pinholes present in the surface of the metallic powder.
As a result of various studies made to develop a method of increasing the stability of metallic powder against oxidation, the present inventors have found that the surface state of metallic powder has a very close relation with the stability of the differential thermal curve of the powder. FIG. 1 is an electron photomicrograph (30,000X) of a known type of magnetic metallic magnetic powder whose differential thermal curve varies at between 20.degree. and 70.degree. C. Obviously, the powder comprises relatively smooth-surfaced acicular particles overlapping each other. On the other hand, as shown in the electron photomicrograph of FIG. 2, a magnetic metallic powder whose differential thermal curve does not change up to a temperature of at least 80.degree. C. comprises acicular particles which are characterized by high surface asperity and a smaller number of pinholes that are generally understood to make magnetic metallic powder less stable against oxidation. This finding which states to the effect that fewer pinholes contribute to an improved heat stability of the differential thermal curve of a particular magnetic metallic powder, and hence an improvement in its stability against oxidation, has led to the accomplishment of the present invention.